This invention relates generally to fiber bed mist eliminators, and fiber beds and collecting media therefor.
Fiber bed mist eliminators have wide industrial application in the removal of aerosols from gas streams. The generation of aerosols (“mist”) in gas streams is common in the course of manufacturing processes. Aerosols can be formed, for instance, as a result of mechanical forces (e.g., when a flow including a liquid runs into a structure) that atomize a liquid. Cooling of a gas stream may result in the condensation of vapor to form a mist, and chemical reactions of two or more gases may take place at temperatures and pressures where the reaction products are mists. However the aerosol comes to be in the gas stream, it can be undesirable to inject the aerosol into other processing equipment because the aerosol may be corrosive or otherwise lead to damage or fouling of the processing equipment. Moreover, it can be undesirable to exhaust certain aerosols to the environment. Some of the more frequent applications of fiber bed mist eliminators include removal of acid mists, such as sulfuric acid mists, in acid manufacturing, removal of plasticizer mists in the manufacture of polyvinyl chloride floor or wall coverings and removal of water-soluble solid aerosols from the emissions of ammonium nitrate prill towers. In these various applications, fiber bed mist eliminators may achieve separation efficiencies of 99% or greater depending upon, among other things, the thickness of the fiber bed.
It is generally known that fibers made of various materials may be used to construct fiber beds for fiber bed mist eliminators. The fiber bed is designed to collect fine liquid mist and soluble solid particles entrained in a moving gas stream and drain them through the structure of the bed. Typically, beds of collecting fibers are associated with metal wire screens or similar external support structures. The combination of a bed of collecting fibers and external support structure is known as a fiber bed assembly. As used hereinafter, fiber bed refers to that portion of the fiber bed assembly apart from any such support structure. Fiber beds may be formed by packing bulk fiber between two opposing support screens (bulk-packed beds), pre-forming a tube of fiber bed material, or winding a roving made of fibers around a cylindrical support screen (wound beds). Although not limited to such a configuration, fiber bed assemblies are most often configured in the form of a vertical cylinder. Cylindrical fiber bed assemblies permit a high effective fiber bed surface area in a minimum of space. Flat fiber bed assemblies on the other hand, find particular application for smaller gas flows.
In operation, a horizontal stream of gas containing a liquid and/or wetted soluble solid aerosol is made to penetrate and pass through the fiber bed of the fiber bed assembly. The fibers in the fiber bed capture the aerosol in the gas by the mechanisms of impaction, interception, and Brownian diffusion. The captured aerosol coalesces on the fibers to form droplets of liquid in the fiber bed. The moving gas urges the droplets to move toward the downstream face of the fiber bed where the captured liquid exits the fiber bed and drains downward under the force of gravity.
The fibers which make up the fiber bed may be made from a variety of materials. Materials utilized to make bed fiber include, for example, metals such as stainless steel, titanium, etc., fibers of polymeric materials such as polyesters, polyvinylchloride, polyethylene terphthalate, nylons, polyethylene, polypropylene etc., and glass. In applications where corrosive conditions and/or high temperatures are encountered, long staple, chemical grade glass fibers have found particularly widespread use in fiber beds of fiber bed mist eliminators. Fibers ranging in diameter from 5 microns or less to more than 200 microns, as well as combinations of fibers of varying diameters, have been used in fiber beds. The bulk density of prior art fiber beds ranges from about 5 lb/ft3 (80 kg/m3) to greater than 20 lb/ft3 (320 kg/m3), while fiber bed thickness ranges from about 0.5 to about 6 inches (1 to 15 cm) or more, depending upon the desired separation efficiency.
In order for a fiber bed to function effectively, the bed must be mechanically stable. A mechanically stable fiber bed is one which will retain its structural integrity without substantial shifting of the fibers relative to adjacent fibers when exposed to the forces exerted by the gas being treated and the captured and draining liquid during aerosol collection. If mechanical stability is not maintained, the performance characteristics of the bed will be unfavorably altered. In a fiber bed lacking mechanical stability, the moving gas stream forces the fibers to shift substantially, causing the liquid ladened fibers in some portions of the fiber bed to mat or felt while increasing the void space between adjacent fibers in other portions of the fiber bed. Matted portions of the fiber bed are more resistant to both the flow of gas and the drainage of captured liquid. Captured liquid which is unable to drain is often reentrained by the moving gas stream, resulting in reduced separation efficiency. Also, the pressure drop across matted portions of an unstable fiber bed is increased. On the other hand, in those portions of a mechanically unstable fiber bed where fiber shifting has increased the average void space between adjacent fibers, macroscopic pores or channels form which reduce separation efficiency by allowing the aerosol containing gas to pass through the fiber bed without sufficient contact with the collecting fibers. In bulk-packed and wound fiber bed assemblies, mechanical stability is largely dependent upon the bulk density of the fiber bed. In these conventionally constructed fiber bed assemblies, a fiber bed bulk density within the range disclosed by the prior art typically provides sufficient contact between adjacent fibers to prevent substantial movement of the fibers when exposed to the forces exerted by the moving gas stream. In bulk-packed fiber beds, density of the fiber bed and resistance to fiber movement is maintained by the reactive compressive force applied against the packed fibers by the two opposing support screens. In preformed fiber bed tubes, the materials may be needled punched or heat formed so that there is significant fiber entanglement or fiber bonding to strengthen the overall bed. Preformed fiber bed tubes have to be reset or adjusted overtime because the fibers shift within the bed. In wound fiber beds, density of the fiber bed necessary to provide mechanical stability is a result of several factors including the tension in the roving as it is wound around the cylindrical support screen and compression of the fiber bed by a wire screen or similar structure which may be wound on the cylinder adjacent to the exterior surface of the fiber roving.
However, bulk density cannot be increased indiscriminately to achieve mechanical stability. If the bulk density of a fiber bed is increased too much, the bed will be prone to flooding. An effective fiber bed is a relatively open structure that allows free gas flow and liquid drainage even when the collecting fibers are coated with collected liquid. There must be sufficient void space between adjacent fibers in the bed so that collected liquid is not able to bridge the space between adjacent fibers to such an extent that the adherence of collected liquid to the surface of fibers prevents the liquid from draining.
A measure of the open space in a fiber bed is void fraction which is defined by the bulk density of the fiber bed and the average density of the fiber material according to the following formula:fiber bed void fraction=1−[fiber bed bulk density/average fiber material density]Fiber beds typically have a void fraction of greater than about 0.89.
It is generally known that the thickness of a fiber bed can be decreased without a loss in separation efficiency by decreasing the average fiber diameter of the fiber material comprising the fiber bed. However, for bulk-packed and wound fiber beds comprising fibers having an average diameter of less than about 5 microns, when the bulk density is high enough to ensure mechanical stability, the resulting void fraction is so low that the bed tends to flood under typical operating conditions. Also, it has been found that conventional thin wound beds are inherently less uniform. A flooded bed is a fiber bed in which captured liquid largely fills the void spaces between adjacent fibers in the fiber bed. A flooded fiber bed is much like the matted portions of an unstable fiber bed. The captured liquid in a flooded fiber bed cannot properly drain and instead may be reentrained in the moving gas stream at the downstream face of the fiber bed. Furthermore, the pressure drop across a fiber bed assembly is increased when the fiber bed is flooded. If a pressure differential across the fiber bed sufficient to overcome the force of adhesion and dislodge the collected droplets from the fibers is employed, the collected liquid is blown from the downstream face of the fiber bed where it is reentrained by the gas stream resulting in low separation efficiency and increased operating cost.
In order to prevent a fiber bed comprising small average diameter fibers from flooding, the specific fiber surface area, expressed as the area of fiber per unit volume of the fiber bed, may be decreased by decreasing the bulk density of the bed (i.e., increasing the void fraction). However, if the bulk density of a bulk-packed or wound fiber bed comprising fibers having an average diameter less than about 5 microns is reduced to a value sufficient to avoid flooding, such fiber beds lack the mechanical stability necessary to withstand the forces exerted by the moving gas stream. As a result, the moving gas stream causes the fibers to shift substantially resulting in the fiber bed matting and/or channeling as previously described. Therefore, in practice, conventionally constructed high efficiency fiber bed assemblies comprise fiber beds 2 to 6 inches (6 to 15 cm) thick constructed of fibers having an average fiber diameter between 5 and 15 microns and having a bulk density between 5 and 15 lb/ft3 (80 and 240 kg/m3).
In contrast to fiber beds used in mist eliminators, some other types of gas filters, such as baghouse, clean room and breathing filters, may successfully utilize glass fibers with average diameters below 5 microns and may even include fibers having a diameter less than 1 micron. However, these types of gas filters are distinguished from fiber bed mist eliminators in that they are typically designed to utilize pore and surface filtration in collecting solid particulates or only relatively small amounts of liquid aerosols. If used to collect liquid aerosols, they easily flood at the liquid loading rates typically encountered in fiber bed mist eliminators. By comparison, fiber bed mist eliminators are designed to allow comparatively large quantities of liquid entrained in a moving gas stream to penetrate the fiber bed where the liquid is captured and continuously drained. Therefore, a solution to the problems associated with utilizing small diameter fibers in fiber beds of fiber bed mist eliminators is neither shown nor suggested by the prior art concerned with such other gas filters.
Reentrainment of collected liquid by the moving gas stream at the downstream face of a fiber bed is often a problem in fiber bed mist eliminator applications, especially in operations characterized by high liquid loading rates or high gas stream velocities. Satisfactory solutions to this problem have included combining a layer of primary filtration fibers and a layer of drainage fibers to form a fiber bed. The drainage layer is downstream of the primary filtration layer and usually comprises fibers with a larger average diameter than those fibers comprising the primary filtration layer.
Despite their success, fiber bed assemblies of the prior art have several disadvantages resulting from shortcomings in the fiber bed. Wound fiber bed assemblies are typically formed using a fiber roving which is roughly cylindrical in shape. The roving is wound around a cylindrical forming screen and back and forth along the length of the screen. This requires skill and the appropriate machinery. Even if the winding is executed correctly, the resultant wound fiber bed may have significant differences in gas flow over the surface area of the bed. These variations are a result of the inherent difficulty of forming a uniform surface on a cylinder using a cylindrical roving. Normal variations in the roving material cause the roving to flatten to different degrees, which affects the uniformity of the fiber bed formed by the roving. The non-uniformity is particularly observed in wound fiber beds having smaller thicknesses.
Another disadvantage associated with bulk-packed and wound fiber bed assemblies is the need to remove the assemblies from the mist eliminator and return them to the manufacturer when the fiber bed needs replacement. Aside from the inconvenience of transporting the heavy support structure, an interchangeable spare fiber bed assembly must be available for the fiber bed mist eliminator to be returned to service and avoid extensive down time. Field replacement of the fiber bed is hampered by difficulties in assembling the fiber bed. Considerable skill and time are required to properly pack new bulk fiber between opposing support screens so that inhomogeneities in the fiber bed are minimized, while winding fiber onto a cylindrical support structure requires extensive machinery. Precast or preformed fiber bed sections have been used by some manufacturers but these have suffered from leakage at joints or settling, requiring extra field maintenance and adjustment. This reduces the productivity of the plant in which the fiber bed is used because the plant must be shut down to carry out the maintenance and/or adjustments.
A third disadvantage is that because fibers having an average fiber diameter of less than about 5 microns cannot be used effectively in constructing a conventional fiber bed without additional processing (e.g., needle punching), fiber bed thickness in applications requiring high separation efficiency cannot be reduced. If thinner high efficiency fiber beds were possible in severe industrial process environments, fiber beds could be configured into shapes that maximize the fiber bed surface area in a given volume available for a fiber bed mist eliminator. This would be analogous to the dry filter art where thin filter papers and felts allow high surface area filter forms via pleating. Thinner, high efficiency fiber beds having increased fiber bed surface area would allow the operating cost of fiber bed mist eliminators to be decreased by decreasing the pressure drop across the fiber bed.